Accurately measuring fluid flow in systems of moving gas or air has always been a difficult task to achieve.
The most common approach to measuring fluid flow entails first determining the differential pressure within a system, often referred to as differential velocity pressure since the pressure differential referred to is the difference between the static and total pressure within the moving fluid system. After determining differential velocity pressure of the moving fluid system, it follows that the velocity and flow volume can be determined mathematically.
As a practical matter though, the real problem in measuring fluid flow in a duct is that the velocity profile across the duct or conduit is almost always nonuniform and not predictable. This is especially true about the sides and corners of ducts or conduits.
To accurately measure flow, one method known entails traversing the cross sectional area of the duct with a differential pressure pitot device and to take multiple readings at numerous positions across the duct. These multiple readings are then appropriately averaged and the velocity of the fluid flow can be determined therefrom. This approach, however, is more of a laboratory method of determining fluid flow, and does not lend itself to an in-line commercial or industrial instrument.
There are commercially available primary devices that are known today for measuring air flow. For example, see the disclosures found in U.S. Pat. Nos. 3,981,193; 4,036,054; and 3,785,206. These primary devices include an array of static and total pressure pitots arranged in a duct so as to yield an average flow across the cross sectional area of the duct. For the most part, such primary devices are not very accurate. Although they use an averaging approach, the velocity profile about the sides and corners of the duct is nonuniform and consequently substantial error is likely to be introduced, resulting in inaccurate flow measurement.